Platelet-rich plasma (hereafter also referred to as “PRP”) is plasma containing a large number of platelets. Whole blood, which contains corpuscle components, contains approximately 95% red blood cells, 3% white blood cells, and approximately 1% platelets. On the other hand, platelet-rich plasma contains a higher proportion of platelets. There is no specific definition of the proportion of platelets in PRP. Taking into consideration that the proportion of plasma in whole blood is, in general, approximately 55%, the proportion of platelets in plasma from which the corpuscle components have been removed is assumed to be approximately 2%. PRP contains a clearly higher proportion of platelets than this approximate 2%.
PRP is obtained by centrifuging whole blood. In detail, first, red blood cells are separated from the whole blood by weak centrifugation to obtain plasma. This plasma contains white blood cells and platelets. Then the plasma is further subjected to strong centrifugation. As a result, platelets are concentrated in the direction to which centrifugal force is applied (hereafter also referred to as centrifugal direction), leaving almost no platelets in supernatant. Then PRP is obtained by removing the supernatant from the plasma which has undergone strong centrifugation, or by taking out only the predetermined amount of the plasma from the part in the centrifugal direction (lower part) (see Patent Publication 1).
It is known that growth factors, such as PDGF, TGF-beta, and ILGF, exist in alpha granules of platelets. It is noted that these growth factors play an effective part in healing of wound and tissue regeneration. For example, PRP is expected to be used in regeneration medicine, such as periodontal-tissue-regeneration methods. (See Patent Publication 2, Patent Publication 3, and Nonpatent Literature 1).    Patent Publication 1: JP, 2006-78428,A    Patent Publication 2: JP, 2006-232834,A    Patent Publication 3: JP, 2005-278910,ANonpatent Literature 1: “Dental And Craniofacial Applications Of Platelet-Rich Plasma” by Robert E. Marx, Quintessence Publishing Co, Inc.